The present invention relates to a serial interface circuit useful, for example, in communication with supervisory and control equipment in telecommunication systems, including digital transmission systems, digital switching systems, and the like.
In such systems, it is common for a single supervisory and control unit to supervise and control a plurality of other units, such as switching units or trunk line units. Each of the supervised and controlled units is conventionally coupled to the supervisory and control unit by a clock signal line, a data signal line, and a scan signal line, forming a three-wire interface. Data are sent and received serially on the data signal line in synchronization with the clock signal, during intervals designated by the scan signal. The clock and scan signals are supplied by the supervisory and control unit.
A problem with this three-wire interface is the number of wires. When the supervisory and control unit is coupled to many supervised and controlled units, the labor involved in connecting three times that many interface wires during equipment installation, and checking the connections during equipment maintenance, is considerable. In addition, the supervised and controlled units have much wiring of their own, and it may be difficult to find space for three interface wires in locations where the wires will not be affected by crosstalk, and will not cause crosstalk in other signal lines.
It would therefore be desirable to replace the three-wire interface with an interface having fewer wires, such as one of various standard two-wire interfaces. Changing over to one of these standard interfaces, however, would require major alterations in signaling formats and protocols, and corresponding modifications of the circuits that transmit, receive, and process the interface signals. Telecommunication system operators are understandably reluctant to make such far-reaching changes in equipment carrying high volumes of public telephone or data traffic.